This invention relates generally to semiconductor device fabrication, and more particularly to the use of phase shift masks (PSM""s) in conjunction with such fabrication.
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC""s with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC""s have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate in sub-wavelength lithography. The resulting features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. For example, as two mask patterns get closer together, diffraction problems occur. At some point, the normal diffraction of the exposure rays start touching, leaving the patterns unresolved in the resist. The blending of the two diffraction patterns into one results from all the rays being in the same phase. Phase is a term that relates to the relative positions of a wave""s peaks and valleys. In FIG. 1A, the waves 102 and 104 are in phase, whereas in FIG. 1B, the waves 106 and 108 are out of phase.
One way to prevent the diffraction patterns from affecting two adjacent mask patterns is to cover one of the openings with a transparent layer that shifts one of the sets of exposing rays out of phase, which in turn nulls the blending. This is shown in FIGS. 2A and 2B. In FIG. 2A, the mask 202 causes an undesirable light intensity as indicated by the line 204. In FIG. 2B, adding the phase shifter 206 to the mask 202 causes a desirable light intensity as indicated by the line 208. This mask 202 in FIG. 2B with the phase shifter 206 added is a phase shift mask (PSM), which is a special type of photomask.
A typical photomask affects only one of the properties of light, the intensity. Where there is chromium, which is an opaque region, an intensity of zero percent results, whereas where the chromium has been removed, such that there is a clear or transparent region, an intensity of substantially 100 percent results. By comparison, a PSM not only changes the intensity of the light passing through, but its phase as well. By changing the phase of the light by 180 degrees in some areas, the PSM takes advantage of how the original light wave adds to the 180-degree wave to produce zero intensity as a result of destructive interference.
PSM""s have gained increased popularity among manufacturers as the feature sizes they are tasked with printing become smaller, and the topography over which these features must be printed becomes more varied. PSM""s offer their customers the opportunity to greatly improve the resolution capability of their steppers. This allows them to print smaller feature sizes using the same equipment and processes.
One particular type of PSM is referred to as an alternating PSM. The PSM of FIG. 2B was one example of an alternating PSM. In an alternating PSM, closely spaced apertures are processed so that light passing through any particular aperture is 180 degrees out of phase from the light passing through adjacent apertures. Any light that spills over into the dark region from the two edges that are out of phase will destructively interfere. This reduces the unwanted exposure in the center dark region.
FIG. 3 shows another example of an alternating PSM, and more specifically, a single-trench alternating PSM 300. The PSM 300 includes two layers, a chromium layer 302, and a quartz layer 304. The chromium layer 302 is the same type of layer typically found in other, non-PSM photomasks, in which light is exposed therethrough to an underlying semiconductor wafer. Clear regions within the chromium layer 302 allow light to pass through, whereas opaque regions within the chromium layer 302 prevent light from passing through. The clear and opaque regions are arranged to correspond to a desired semiconductor design, or pattern. In the PSM 300, there are clear regions 306A, 306B, 306C, 306D, and 306E.
The quartz layer 304 is more generally a clear or transparent layer, in which single trenches are alternatively added beneath the clear regions of the chromium layer 302 to phase shift light passing through these clear regions. For instance, the alternating clear regions 306A, 306C, and 306E of the chromium layer 302 do not have single trenches beneath them in the quartz layer 304. Conversely, the alternating regions 306B and 306D of the chromium Elayer 302 have single trenches 308A and 308B beneath them in the quartz layer 304. The PSM 300 is an alternating PSM in that only every other clear region of the chromium layer 302 has a phase shifter beneath them in the quartz layer 304. The PSM 300 is a single-trench alternating PSM in that these phase shifters are the single trenches 308A and 308B, as compared to other types of phase shifters, such as double trenches, and so on.
The manner by which the PSM 300 of FIG. 3 can be fabricated according to the prior art is summarized with reference to FIGS. 4A and 4B. In FIG. 4A, the clear regions within the chromium layer 302 are already present, by a process of photoresist patterning, etching the chromium layer 302, and then stripping the remaining photoresist. A new layer of photoresist 402 has been added, such as by a coating process, and patterned to correspond to where the single trenches 308A and 308B of FIG. 3 will be made. In FIG. 4B, the quartz layer 304 is first dry etched, and then wet etched using sodium hydroxide (NaOH) to form the single trenches 308A and 308B. The photoresist layer 402 is then removed by a photoresist strip process to result in the PSM 300 of FIG. 3.
This conventional approach to manufacturing the alternating single-trench PSM 300 has several disadvantages, however. To not damage the quartz layer 304 and/or the chromium layer 302, as well as possibly for other reasons, the patterning of the photoresist resulting in the photoresist layer 402 of FIG. 4A must be accomplished by using laser-beam writing to properly expose the photoresist. This means that more conventional tools, such as e-beam writers, cannot be used to expose the photoresist to result in the photoresist layer 402 of FIG. 4A. Furthermore, phase defects in the PSM 300 are difficult to repair when using the process summarized with reference to FIGS. 4A and 4B.
The depth of the quartz layer 304 after dry etching, and before wet etching, also cannot be accurately measured with the photoresist layer 402 remaining on top of the chromium layer 302, which is problematic to ensure that the phase shift resulting from the PSM 300 is correct. Accurate measurement cannot be accomplished because the photoresist layer 402 is an inaccurate reference from which to measure the depth of the of the quartz layer 304 after dry etching. Furthermore, the wet etching of the quartz layer 304, because it uses NaOH, may cause the photoresist layer 402 to peel, decreasing the likelihood that the single trenches 308A and 308B will be properly fabricated. A different approach to single-trench alternating PSM manufacture, described in U.S. Pat. No. 5,958,630, also suffers from at least some of these problems.
Therefore, there is a need for a process for fabricating a single-trench alternating PSM that overcomes the problems associated with manufacturing such PSM""s in the prior art. Such a process should be able to use equipment other than laser writers for use in photoresist patterning in preparation of etching the trenches in the quartz layer. The process should also enable more easily accomplished repairs of any defects in the PSM, and enable accurate measurement of the depth of the trenches without having to strip, reapply, and re-pattern photoresist. The process should finally avoid damaging photoresist, or otherwise increase the likelihood that the single trenches will be properly fabricated. For these and other reasons, there is a need for the present invention.
The invention relates to fabricating a single-trench alternating phase shift mask (PSM). An opaque layer over a mask layer, which is itself over a transparent layer, of the PSM is patterned according to a semiconductor design. The opaque layer may be a chromium layer, whereas the transparent layer may be a quartz layer. The mask layer and the transparent layer are dry etched through a photoresist layer that has been applied over the opaque layer and patterned according to an alternating PSM design.
The dry etching initially forms single trenches of the PSM. The transparent layer is next wet etched through the mask layer to completely form the single trenches of the PSM, where the photoresist layer has first been removed. The mask layer is dry etched again, where the single trenches of the PSM are initially filled with filler material to protect the single trenches from the dry etching. The filler material is finally removed. The filler material may be photoresist.
The invention provides for advantages not found within the prior art. The patterning of the photoresist layer prior to the dry etching of the mask layer and the transparent layer can be accomplished by e-beam writing, and not only laser beam writing as in the prior art. This is because the mask layer protects the transparent layer from damage that would otherwise result from e-beam writing. Either hydrogen fluoride (HF) or sodium hydroxide (NaOH) can be used to wet etch the transparent layer, as compared to the prior art, which only permits NaOH wet etching. The transparent layer can further be undercut by wet etching with megasonic cleaning. The depth of the single trenches after their initial formation by the first dry etching can be accurately measured, because the photoresist layer is removed prior to wet etching. That is, the opaque layer serves as a reference from which to measure the depth of the single trenches, allowing for accurate measurement.
Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.